The Biomechanical Case for Minimalist Running

A few weeks ago, I gave a talk on the current state of research into minimalist running at a conference of the Canadian Academy of Sport and Exercise Medicine, which involved sifting through a bunch of papers both new and old. I thought I'd share the info from one of my slides, in which I argued that we should be cautious about forming conclusions based on biomechanical data.

First, I showed some graphs that have become very familiar over the past few years. They're from Daniel Lieberman's 2010 Nature paper in which he compared foot strike and collision forces in barefoot and shod runners (the graphs are also available on Lieberman's public webpage on barefoot running). The graphs show the impact force in three conditions: (a) barefoot heel strike; (b) shod heel strike; and (c) barefoot forefoot strike:

What stands out in these graphs isn't the peak force, which is about 2.5 body weights in all three cases. It's the initial sharp, steep spike for the heel strikers. This "impact transient" is worst for the barefoot heel strikers; it's reduced and broadened a bit for heel strikers wearing shoes; and it completely disappears for forefoot strikers. What Lieberman and others argue is that the factor contributing to injury is how steeply the graph rises initially, because that tells you how sudden the jolt through your joints is. In this picture, it looks pretty compelling that heel striking is a no-no. In a sense, these graphs are the foundational texts for the scientific discussion of minimalism — and they look pretty convincing.

What I pointed out in my talk was the following three graphs:

They should look familiar, because they're essentially the same as the graphs from the Lieberman paper, and they're taken under the same conditions (barefoot heel strike, shod heel strike, barefoot forefoot strike). But these graphs come from a 1985 Journal of Biomechanics paper by researchers at Tulane. In other words — as numerous biomechanics researchers have pointed out — the "new" findings suggesting that barefoot is best have been around for decades.

What's really interesting is to look at how this data was interpreted the first time around. First of all, it was clear to them that the shod heel strike looked better than the sharp-peaked barefoot heel strike, leading them to conclude that shoes with cushioned heels were doing a good job. In future shoes, they suggested, "the heel should be made of a substance which provides the maximum shock-absorbing capacity, thus damping the vibration evident in the axial skeleton."

In addition, building on earlier similar work by Cavanagh and Lafortune from 1980, they noted that the largest forces actually occur later in the gait cycle, not at impact. In fact, they say that "approximately 75% of running injuries (tendonitis, shin splints, stress fractures, plantar fascitis and chondromalacia) seem to be due to the high forces occurring at push off, when the forces across the knee and ankle are maximal." And it gets worse: "the mediolateral component of force for midfoot strikers was 3 times greater than that seen in heel strikers." In fact, the Cavanagh and Lafortune paper argues that "midfoot strikers are particularly at risk."

So what's going on here? We have two studies that present identical biomechanical data. One concludes that the best way to prevent injury is to switch to minimal (or no) shoes and land on the forefoot; the other concludes that we need cushioned heels and should avoid midfoot or forefoot strikes. Were we simply too dumb in 1985 to see the obvious truth — or is the data a Rorschacht blot that allows us to impose our existing assumptions on it?

To me, this is a great example of how difficult it is to draw foolproof conclusions from observational data. Biomechanical analysis produces an avalanche of data on the various forces and motions affecting different joints. Which parameters are the most important? If you think barefoot running is the best way to run, your attention is inevitably drawn to the impact transient; if you're focused on how to engineer the best shoe, you notice the mediolateral force. The only way to really determine how different footstrikes affect injury is to directly test it with an intervention study. People like Lieberman have been careful to point this out repeatedly (here's what it says on his barefoot running page: "We emphasize, however, that this hypothesis on injury has yet to be tested and that there have been no direct studies on the efficacy of forefoot strike running or barefoot running on injury."), but that message often gets lost. That's why I was enthusiastic aboutthe pilot intervention study I blogged about a few weeks ago. These are hard studies to do well — but until they're done, we're just going around in circles, looking at the same old data and drawing our own conclusions.

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